If water is clear, why is the ocean blue?

People often mistakenly think that the sea is blue because it reflects the sky, but the truth is that pure water is very slightly blue. It is just so pale that in small quantities it appears clear.

Water is blue because it absorbs red, yellow and green light, but scatters blue light.

Fundamentally, what makes water blue is the fact that it absorbs light that is red, yellow and green, and scatters light that is blue ─ this is because blue light travels as shorter, smaller waves that are scattered more easily (this is essentially the same reason why the sky is also blue). The reason a small amount of water appears clear is because not much light is scattered. In larger bodies of water there are more water molecules for light to collide with, resulting in more blue light being scattered. Think of it like throwing a bucket-full of tennis balls against a tree from a distance: the chances are that most balls will miss the tree and keep going. But if you stand in a forest and throw tennis balls, more balls will hit trees and bounce back. It is exactly the same when it comes to light hitting water molecules: if there are more water molecules in the way, more light particles will collide with them and be scattered, making the water appear a deeper shade of blue.

In deeper water more blue light is scattered because there are more water molecule in the path of the light. This results in a deeper shade of blue.

There are of course other things that can change the colour of water. Lots of mud and sediment in rivers can make them appear brown; the same is true in shallow areas of the ocean, especially if a storm has stirred up lots of sediment from the seabed. Algae can turn the water green, because they contain the green pigment chlorophyll. A few species of algae may even turn the water red because they contain red pigments. These species are often toxic and when they bloom they form what are known as “red tides”, making the ocean appear like it has turned into blood.

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This post is an excerpt from “Do Fish Sleep?: and 38 other ocean mysteries”, by David Aldridge. It is available to buy on kindle here for the reduced price of $1.46 (99p) until the end of January. To read it, you do not need to own a Kindle device, just the Kindle App which is a free download for smartphones or tablets. It can also be read with“Kindle Cloud Reader” on a PC or Laptop.